Same issue, did you have any success? We have hosted feature layers in AGOL. The "Rebuild Spatial Index" option just hangs, and once it's hung, it is never available again. It will report a "Creating index: Indexes may take a moment depending upon the amount of data." message, one of which I've had now for a month.
Looking under the hood, the Feature Layer Definition JSON shows an "updateLayerDefinitionStatus" of "InProgress", but I've found nothing online for that parameter.